The magnet assemblies used in Magnetic Resonance Imaging (“MRI”) must create a very strong static primary magnetic field with precise geometry. A dynamic gradient magnetic field is then added over the primary magnetic field. Particular imaging sequences used to create MRI images require application of the gradient field with rapid variations in amplitude.
Certain static-field magnets use ferromagnetic poles projecting toward a patient receiving space. The flux in the patient receiving space is fairly uniform near the central axis of the pole. However, at the edges of the poles, remote from the central axis, the field tends to bulge outward away from the central axis, thereby altering the flux geometry.
The problem of the bulging of the magnetic field at the edges of the magnet has been addressed by adding shims to adjust the shape of the magnetic field flux. A shim is a piece of metal, typically a low carbon steel or other ferromagnetic material, that projects from the poles, typically near the edges of the poles. For example, a shim on a cylindrical pole may be partially in the form of a circular rim projecting toward the patient. This added magnetic material changes the shape of the poles, and therefore the shape of the field the magnet creates. The result is a change in magnetic flux profile at the edges and a more uniform magnetic field.
The gradient fields are commonly added by gradient coils disposed adjacent to the pole surfaces. Eddy currents can be induced in the ferromagnetic poles as the current in the gradient coils is varied. The eddy currents, in turn, induce their own magnetic fields. This effect is undesirable because it lengthens the time it takes for the gradient flux to stabilize at the desired amplitude, and creates undesirable variations in the magnetic field which cause a loss of image quality.
One structure to suppress eddy currents in magnetic assemblies is disclosed in U.S. Pat. No. 5,124,651 to Danby, et al. (“the '651 patent”), the disclosure of which is incorporated by reference herein. The preferred embodiments of the '651 patent provide pole faces resistant to eddy currents. Each pole face includes a plurality of narrow, ferromagnetic solid rods positioned side-by-side to each other. These rods are bonded to one another by an epoxy resin or other non-conductive bonding material, thereby creating thin cylinders. The thin cylinders are mounted on the tips of the poles so that the axes of the cylinders, and the axes of the individual rods, extend generally parallel to the polar axis of the magnet. The rods are electrically insulated from one another by the non-conducting bonding material. This eliminates eddy currents directions transverse to the polar axis to the magnetic flux while providing enough ferromagnetic material to facilitate magnetic field flux in directions parallel to the polar axis. In certain embodiments according to the '651 patent, a shim ring can be formed by rods at the edges of the cylinder extending beyond the remainder of the cylinder.
While this structure is effective in suppressing eddy currents in magnetic assemblies, there is still need for further improvements.